This application is based on application No. 11-164729 filed in Japan, the contents of which are hereby incorporated by reference.
The present invention pertains to a new liquid crystal composition and to a heat-sensitive recording medium that uses this composition, wherein the heat-sensitive recording medium allows information to be written to it when particular heating conditions are present, and also exhibits reversibility, such that this information can also be erased.
In recent years, in conjunction with the increased attention being paid to reduced resource consumption and recycling, it has been desired to enable the re-use of recording media such as paper. Technological development efforts toward this end have focused on rewritable heat-sensitive recording materials onto and from which data can be recorded and erased by means of a heating means, such as a thermal head.
Known reversible heat-sensitive recording materials include a leuco pigment/coupling control agent, an organic low-molecular weight or polymer resin matrix, or a polymer cholesteric liquid crystal composition.
A leuco pigment/coupling control agent exhibits color when the lactone ring that is contained in the leuco pigment molecule opens and loses color when the ring closes. A lactone ring opens when it is rapidly cooled after its temperature is increased, and closes when it is gradually cooled. Such a leuco pigment/coupling control agent is applied onto a sheet, so that writing may be performed using a thermal head, and the information is erased when the sheet passes through heat rollers.
Known types of organic low-molecular weight or polymer resin matrix employ BA (behenic acid) as an organic low-molecular weight compound or PVCA (polyvinyl chloride acetate) as a polymer compound. These materials can alternate between a transparent state and a light scattering state depending on the heating temperature, and are maintained in either state when cooled. When this type of material is applied onto a sheet, information can be written on it using a thermal head.
A known polymer cholesteric liquid crystal composition includes a cholesteric liquid crystal compound comprising polymerized vinyl compounds having a cholesteric liquid crystal compound on the surveying chain. The display color of this composition can be changed and fixed by heating the composition to a certain temperature above its crystallization point and then rapidly cooling it.
Certain compounds are known to comprise a low-molecular weight cholesteric liquid crystal, as shown in Adv. Mater., 1997, 9(14), pp. 1102-1104. With these materials, the display color may be changed and fixed by heating the compound to a temperature above the melting point and then cooling it rapidly.
Using a leuco pigment/coupling control agent described above, the color displayed is determined by the leuco pigment used, and display of multi-color images is not possible. Since an organic low-molecular weight/polymer resin matrix performs display based on the transmission and scattering of light, multi-color display is not possible with that material either. With polymer cholesteric liquid crystal, while it is possible in principle to change the display color based on the heating temperature, it takes up to several minutes to change color, which has posed a major obstacle to putting it to practical use. Using low-molecular weight cholesteric liquid crystal, the display color may be changed based on the heating temperature, but it has the problems that the range of temperature in which the color may be changed is small, and it has poor reproducibility.
The object of the present invention therefore is to provide a more appropriate new liquid crystal compound and composition as a reversible heat-sensitive recording material. Another object of the present invention is to provide a reversible heat-sensitive recording medium that (i) is capable of performing color display at a high speed, (ii) can change the display color in a wide range based on easy temperature control, and (iii) has good reproducibility with little chromatic unevenness.
In order to attain these objects, the present inventors have found liquid crystal compositions having an enthalpy xcex94Hch that, during its transition from the solid phase to the cholesteric phase, is constant at A (0xe2x89xa6Axe2x89xa615 mJ/mg) when the temperature increases during differential thermal analysis or differential scanning calorimetry. Thus, the present inventors have found that by using this liquid crystal composition as a recording layer, the temperature range of the liquid crystal composition in which it exhibits color is increased and thermal writing by means of a thermal head, etc., becomes practical.
The liquid crystal composition pertaining to the present invention comprises a liquid crystal composition that includes a cholesteric liquid crystal compound that exhibits a cholesteric phase at temperatures higher than room temperature, wherein the liquid crystal compound reflects light in the visible light range in accordance with the temperature, and solidifies in the reflective state when it is rapidly cooled, and wherein the enthalpy xcex94Hch of the liquid crystal compound during its transition from the solid phase to the cholesteric phase is constant at value A (0xe2x89xa6Axe2x89xa615 mJ/mg) when the temperature increases during differential thermal analysis or differential scanning calorimetry. Such a liquid crystal composition has a large temperature range in which it exhibits color and is appropriate as a recording material used in a heat-sensitive recording medium.
The heat-sensitive recording medium pertaining to the present invention comprises, for example, a sheet-like support member (base layer) and a recording layer. The recording layer comprises a liquid crystal composition including a cholesteric liquid crystal compound that exhibits a cholesteric phase when it is within a certain temperature range and reflects light in the visible light range in accordance with the temperature, and that solidifies in the reflective state when it is rapidly cooled from that temperature. Specifically, the recording layer comprises a liquid crystal composition whose enthalpy xcex94Hch during its transition from the solid phase to the cholesteric phase is constant at A (0xe2x89xa6Axe2x89xa615 mJ/mg) when the temperature increases during differential thermal analysis or differential scanning calorimetry.
By having such a recording layer, the heat-sensitive recording medium pertaining to the present invention can display a desired color when it is heated to the temperature required, based on the information to be recorded, and is capable of multi-color display. In addition, the displayed information may be erased when the medium is reheated. Further, various display colors are available, and little chromatic unevenness is exhibited. The temperature range in which the color may be changed is also wide and temperature control is easy.
The representative liquid crystal compounds that may be used in the present invention consist of the compounds having the chemical structures (I) through (VII) shown below. For the liquid crystal composition, these compounds may be used on their own or as a mixture of two or more, as long as the single compound or the mixture exhibits an enthalpy xcex94Hch at constant A (0xe2x89xa6Axe2x89xa615 mJ/mg) during its transition from the solid phase to the cholesteric phase when the temperature increases during differential thermal analysis or differential scanning calorimetry. Other ingredients, such as liquid crystal compounds other than those having the chemical structures (I) through (VII), may also be added.
When two compounds are mixed, the composition may be selected in the range between 9:1 to 1:9 by weight ratio, and in particular, a weight ratio between 1:1 and 3:1 is preferred. Furthermore, compounds that include a cholesterol radical are preferred, and those including two cholesterol radicals are most preferred. 
where (a) and (b) are integers such that (a+b) is 5 through 20. 
where (c) indicates an integer 5 through 20.
where (d) and (e) are integers such that (d+e) is 5 through 20. 
where (f), (g) and (h) are integers such that (f+g+h) is 5 through 20.
where (i), (j) and (k) are integers such that (i+j+k) is 5 through 20, excluding the case where (j) is 0.
where (l) indicates an integer 5 through 20.
where (m) indicates an integer 5 through 20.
Specific examples of the chemical structures (I) through (VII) above include the following compounds having the structures (A) through (N). 
The compounds having the chemical structure (I) may be obtained by (1) having 1-bromocarboxylic acid having an alkyl chain corresponding to the length of the target compound and di-lithium diyne react with each other in an appropriate catalyst, and (2) having the compound thus obtained react with cholesterol.
The compounds having the chemical structure (II) may be obtained by having cholesterol and a dicarboxylic acid derivative having an alkyl chain corresponding to the length of the target compound react with each other in an appropriate catalysis.
The compounds having the chemical structure (III) may be obtained by following these steps: (i) perform Williamson synthesis of hydroquinon and carboxylate halide having an alkyl chain corresponding to the length of the target compound, (ii) perform hydrolysis of the product of the synthesis, and (iii) have the compound thus obtained and cholesterol condense in an appropriate catalyst.
The compounds having the chemical structure (IV) may be obtained by having cholesterol and a dicarboxlyic acid derivative having an alkynil chain corresponding to the length of the target compound react with each other in an appropriate catalysis.
The compounds having the chemical structure (V) may be obtained by having cholesterol and a dicarboxylic acid derivative having an alkynil chain corresponding to the length of the target compound react with each other in an appropriate catalysis.
The compounds having the chemical structure (VI) may be obtained by having cholesterol choloroformate and a dialcohol derivative react with each other in an appropriate catalyst.
The compounds having the chemical structure (VII) may be obtained by having cholesterol and a carboxylic acid derivative having an alkyl chain corresponding to the length of the target compound react with each other in an appropriate catalyst.
In the heat-sensitive recording medium pertaining to the present invention, the recording layer is mounted on the front surface and/or rear surface of the support member. If the support member has a specific color, display having a background of a specific color may be performed. In particular, if the support member is black, good display color may be obtained. Further, if the surface of the support member that comes into contact with the recording layer is essentially flat, the helical axes of the liquid crystal composition will be precisely oriented in the direction perpendicular to the support member, enabling display with a high reflection rate.
Furthermore, in the heat-sensitive recording medium pertaining to the present invention, spacers and polymer materials may be used in order to maintain the thickness of the recording layer. It is also acceptable to close off the edges of the recording layer using a sealing material. Protective film may be applied in order to protect the recording layer. The protective film may comprise deposition film or a polymer material. The protective film protects the recording layer from receiving external mechanical or chemical forces.
Moreover, in the heat-sensitive recording medium pertaining to the present invention, part of the recording layer may be made a non-reversible heat-sensitive recording medium area, such that specific information may be written to this area in advance. For example, if a company name or a decorative pattern is written there in advance, the recording medium may be used as a specifically formatted recording medium.